s390/setup: fix memblock usage

When converting from bootmem to memblock I missed a subtle difference:
the memblock_alloc() functions return uninitialized memory, while the
memblock_virt_alloc() functions return zeroed memory.

This led to quite random early boot crashes.

Therefore use the correct version everywhere now.
Hopefully.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>